Cannot delete eol arounded by multibyte characters when auto-format is set

Hi,

I have trouble in using auto-format in vim to format Chinese characters. The eol in a formatted paragraph can not be deleted in insert mode when formatoptions is set to "tcqmBa".

To reproduce the trouble:

1. vim --clean

2. insert the following text.

你好，你是谁？你从哪里来？你要到哪里去？

你好，你是谁？你从哪里来？你要到哪里去？

你好，你是谁？你从哪里来？你要到哪里去？

你好，你是谁？你从哪里来？你要到哪里去？

3. :set formatoptions=tcqmBa

4. visual select the four-lines text and press "gq" to format.

5. then the formatted text is supposed to have 2 lines.

6. move the cursor to the beginning of the second line and press i to insert

7. press backspace or <c-w> <c-h>. the eol in front of the cursor can not be deleted.

Expected behavior:

the eol  would be deleted so that the characters in the first line is able to be deleted.

vim version: v9.0.1321

OS: Debian GNU/Linux 11 (bullseye)

Is this problem a bug or could someone give me some suggestions to achieve the goal? Thanks a lot!

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